In a classical combustion apparatus for producing a high-velocity flame jet, a fuel and an oxidizer are combined in a combustion chamber. The combined fuel and oxidizer are then ignited to produce combustion gases, and these gases are then accelerated through a nozzle. FIG. 1 is a cross-section view that illustrates a typical example of a conventional combustion device 10, having a housing 11 containing a combustion chamber 12. The combustion chamber 12 communicates with a nozzle 13 and an exit passage 14. An oxidizer, usually gaseous oxygen, is introduced into the combustion chamber 12 through an oxidizer orifice 15. Fuel, either liquid or gas, enters the combustion chamber 12 through a fuel inlet 16 to mix with the oxidizer flow from the oxidizer orifice 15. Ignition, often provided by a spark-plug (not shown), occurs to form an intense flame in the combustion chamber 12. The width and length of the combustion chamber 12 are sized to provide essentially complete combustion of the fuel and oxidizer. Prior to entry into the nozzle 13, the velocity of the hot combustion products is quite low. The combination of a restricting cross section of the nozzle 13 with an expanding cross section of the exit passage 14 serves to greatly accelerate the combustion gasses. This structure is termed a de Laval nozzle.
Due to the extreme heat generated in the combustion device 10, external cooling is required. An outer shell structure 20 is spaced a small distance away from the housing 11, forming an annular coolant passage 21. Water passes into the annular coolant passage 21 through a coolant inlet 22, exiting through a coolant outlet 23. The requirement for water cooling complicates the structure and reduces thermal efficiency, since much of the energy generated by combustion is lost in the form of heat.